Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio-frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are also referred to as “1xRTT (1x) networks,” which stands for “Single Carrier Radio Transmission Technology”. These networks (referred to herein as “CDMA networks”) typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. As a general matter, sectors are used herein as examples of wireless coverage areas. When a mobile station is positioned in a sector, it can communicate over the RF air interface with the base station, and, in turn, over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier. Note that base stations may provide service in a sector on one carrier, or on more than one, and further that a given instance of a given carrier in a given sector may be referred to herein as a “sector/carrier.”
In general, a pilot channel functions to alert mobile stations in a given sector of the presence of a service-providing base station. Typically, the pilot channel conveys a value known as a pseudorandom number (PN) offset, which identifies the sector; in particular, by being offset from CDMA system time by a certain amount of time, the pilot channel conveys the PN offset. Mobile stations generally use the pilot channel to coherently detect and demodulate the signal on a sector/carrier.
In a typical wireless network, a mobile station can communicate with a number of “active” sectors or coverage areas at a time, typically known as the “active set” for that mobile station. Depending on the system, the number of active sectors can be up to three or six (currently), as examples. The mobile station receives substantially the same information from each of the coverage areas in the active set and, on a frame-by-frame basis, selects the best signal to use. The mobile station, one or more base stations, and/or a base station controller maintains in memory a list of the coverage areas in the mobile station's active set.
In existing systems, to facilitate a determination of which sectors should be in the mobile station's active set, all base stations emit the pilot channel signal, typically at a power level higher than other forward-link signals. A mobile station constantly measures the strength of each pilot that it receives, and notifies at least one base station when pilot strength for various coverage areas falls above or below designated thresholds. The base station, may, in turn, provide the mobile station with an updated list of active pilot signals (i.e. an updated active set).
In a CDMA system configured as above, a link from the base station to the mobile station is called a forward link and a link from the mobile station to the base transceiver station is called a reverse link. All forward links from a base station have the same PN offset. The PN offset is transmitted via the pilot channel signal, as one of the forward channels. A mobile station continuously monitors the pilot channel signal in order to obtain a good quality signal on the forward channel. A mobile station located within the service coverage of the base station uses the pilot signal for synchronization. A mobile station can acquire the timing of the forward CDMA channel from the pilot signal and obtain the phase reference for coherent demodulation.
The mobile station can simultaneously communicate with a plurality of base stations while differentiating the code. The above function is known as a soft handoff. That is, each of the base stations transmits a pilot signal having its own PN offset, so the mobile station searches for the timing location from which the strongest pilot signals are received. After the mobile station receives the pilot signal from the base station, the base station continues to instruct the mobile station to search for transmitted pilot signals from other base stations, as well as using a particular timing location for each base station.
At this time, the base station provides the mobile station with a search window to search for the pilot signals. A search window is the amount of time, in terms of chips, that a mobile station will search for a pilot channel, where a chip is the unit of code spreading for CDMA (one chip is approximately 0.8 μs in duration). When the mobile station is initially connected to the base station, the base station transmits a search-window size corresponding to the active set, to the mobile station.